As far as I know they have no appreciable effect on accuracy. They do have an appreciable effect on the flash that the shooter sees from the muzzle, which isn't really a concern when it comes to someone who's slow firing at targets during a range day.
Which is something I always thought was strange... why have a 14" barrel with a pinned 2 inch flash suppressor, when you could just have a 16" barrel, no flash suppressor, and get the velocity benefits?
Because is a self defense oriented firearm, shorter barrels mean a more compact weapon that is easier to maneuver with. Shorter barrels also usually lead to a larger muzzle flash, hence the flash suppressor.